Silicon nitride is frequently utilized in modern semiconductor fabrication methods. For instance, silicon nitride is an insulative material, and can be utilized to electrically isolate conductive components from one another. Also, silicon nitride is selectively etchable relative to other materials utilized in semiconductor fabrication processes, such as, for example, silicon dioxide, and is can thus be utilized as an etch stop material. Another example use of silicon nitride is for LOCOS (LOCal Oxidation of Silicon). LOCOS comprises growing oxide over field regions of a semiconductor substrate, while not growing the oxide over other regions of the substrate. The other regions of the substrate are typically protected by a thin layer of silicon nitride during the oxide growth.
In many applications of silicon nitride, a silicon nitride layer is patterned into a specific shape. An example prior art patterning process is described with reference to FIGS. 1-2. Referring to FIG. 1, a semiconductor wafer fragment 10 comprises a substrate 12 covered by a pad oxide layer 14, a silicon nitride layer 16, an antireflective coating 18, and a photoresist layer 20.
Substrate 12 can comprise, for example, monocrystalline silicon lightly doped with a p-type dopant. To aid in interpretation of the claims that follow, the term "semiconductive substrate" is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term "substrate" refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Pad oxide 14 is a thin layer (from about 40 to about 50 nanometers thick) of silicon dioxide, and is provided to alleviate stresses that can be caused by silicon nitride layer 16. Pad oxide 14 can be formed by exposing a silicon-comprising substrate 12 to an oxidizing atmosphere.
Silicon nitride layer 16 can be formed over pad oxide 14 by, for example, chemical vapor deposition. A thickness of silicon nitride layer 16 will vary depending on the application of the silicon nitride layer. In LOCOS fabrication processes, silicon nitride layer 16 will typically be provided to a thickness of from about 100 nanometers to about 200 nanometers.
Antireflective coating 18 is a polymer film provided over silicon nitride layer 16 for two purposes. First, antireflective coating 18 absorbs light during photolithographic patterning of photoresist layer 20. Such absorption can prevent light that has passed through photoresist layer 20 from reflecting back into the layer to constructively or destructively interfere with other light passing through layer 20. Second, antireflective coating 18 functions as a barrier to prevent diffusion of nitrogen atoms from silicon nitride layer 16 into photoresist layer 20. It is found that if nitrogen atoms diffuse into photoresist 20, they can alter its sensitivity to light (so-called "poisoning" of the photoresist).
Photoresist layer 20 is provided to form a pattern over silicon nitride layer 16. Photoresist layer 20 comprises a polymer composition which becomes selectively soluble in a solvent upon exposure to light. If photoresist 20 comprises a negative photoresist, it is rendered insoluble in a solvent upon exposure to light, and if it comprises a positive photoresist, it is rendered soluble in solvent upon exposure to light.
Referring to FIG. 2, photoresist layer 20 is exposed to a patterned beam of light to selectively render portions of the photoresist soluble in a solvent, while leaving other portions insoluble. After such exposure, the solvent is utilized to selectively remove portions of photoresist layer 20 and thereby convert photoresist layer 20 into the pattern shown.
Referring to FIG. 3, the pattern from layer 20 is transferred to underlying layers 18, 16 and 14 by an appropriate etch. A suitable etch can comprise, for example, a plasma-enhanced etch utilizing NF.sub.3 and HBr. In subsequent processing which is not shown, antireflective coating layer 18 and photoresist layer 20 can be removed to leave stacks comprising pad oxide 14 and silicon nitride 16 over substrate 12. The stacks can then be utilized for subsequent fabrication processes. For instance, the stacks can be utilized for LOCOS by subsequently exposing wafer fragment 10 to oxidizing conditions to grow field oxide between the stacks. As another example, conductive metal layers may be provided between the stacks, and the stacks utilized for electrical isolation of such metal layers.
The above-described processing sequence requires formation of four distinct layers (14, 16, 18, and 20), each of which is formed by processing conditions significantly different than those utilized for formation of the other three layers. For instance, antireflective coating 18 is commonly formed by a spin-on process, followed by a bake to remove solvent from the layer. In contrast, silicon nitride layer 16 is typically formed by a chemical vapor deposition process. The spin-on and baking of layer 18 will typically not occur in a common chamber as the chemical vapor deposition of layer 16, as processing chambers are generally not suited for such diverse tasks. Accordingly, after formation of silicon nitride layer 16, semiconductor wafer fragment 10 is transferred to a separate processing chamber for formation of antireflective coating 18. The semiconductive wafer fragment 10 may then be transferred to yet another chamber for formation of photoresist layer 20.
A continuing goal in semiconductive wafer fabrication processes is to minimize processing steps, and particularly to minimize transfers of semiconductive wafers between separate processing chambers. Accordingly, it would be desirable to develop alternative fabrication processes wherein fabrication steps could be eliminated.
It has been attempted to pattern silicon nitride layers without utilizing an antireflective coating over the layers. However, such creates complications, such as those illustrated in FIG. 4. Identical numbering is utilized in FIG. 4 as was utilized with reference to FIGS. 1-3, with differences indicated by the suffix "a". A difference between the semiconductive wafer fragment 10a of FIG. 4 and the wafer fragment 10 of FIGS. 1-3 is that antireflective coating 18 is eliminated from the wafer fragment 10a construction. Wafer fragment 10a of FIG. 4 is shown at a processing step analogous to the processing step shown in FIG. 2. Elimination of antireflective coating layer 18 has enabled nitrogen atoms to diffuse from silicon nitride layer 16 into a lower portion of photoresist layer 20a. The nitrogen atoms have altered the photoresist such that regions which should be removed by exposure to a solvent are no longer removable by the solvent. This can render semiconductive wafer fragment 10a unsuitable for the further processing described above with reference to FIG. 3. It would be desirable to develop alternative methods of forming photoresist over silicon nitride which avoid the adverse effects illustrated in FIG. 4.
Traditional silicon nitride layers have stoichiometries of about Si.sub.3 N.sub.4. Silicon enriched silicon nitride layers (i.e., silicon nitride layers having a greater concentration of silicon than Si.sub.3 N.sub.4, such as, for example, Si.sub.4 N.sub.4) have occasionally been used in semiconductor fabrication processes. The silicon enriched silicon nitride was utilized as a layer having a substantially homogenous composition throughout its thickness, although occasionally a small portion of the layer (1% or less of a thickness of the layer) was less enriched with silicon than the remainder of the layer due to inherent deposition problems.
When the silicon enriched silicon nitride layers were utilized in a process such as that shown in FIG. 4 (i.e., a process wherein no antireflective coating layer is provided between the silicon nitride and the photoresist), it was found that good photolithographic patterning of the photoresist could be obtained. However, it was also found that the silicon nitride was difficult to pattern due to a resistance of the silicon nitride to etching. Accordingly, it would be desired to develop alternative methods of utilizing silicon nitride in wafer fabrication processes.
In additional aspect of the prior art, microcircuit fabrication involves provision of precisely controlled quantities of impurities into small regions of a silicon substrate, and subsequently interconnecting these regions to create components and integrated circuits. The patterns that define such regions are typically created by a photolithographic process. Such processing sets the horizontal dimensions on the various parts of the devices and circuits. Photolithography is a multistep pattern transfer process similar to stenciling or photography. In photolithograpy, the required pattern is first formed in reticles or photomasks and transferred into the surface layer(s) of the wafer through photomasking steps.
Inherent in photolithograpy is application and adherence of photoresist materials to underlying substrates. The resist must be capable of adhering to these surfaces through all the resist processing and etch steps. Poor adhesion brings about severe undercutting, loss of resolution, or possibly the complete loss of the pattern. Wet etching techniques demand a high level of adhesion of the resist film to the underlying substrates.
Various techniques are used to increase the adhesion between resist and a substrate such as, a) dehydration baking prior to coating; b) use of hexamethyldisilazane (HMDS) and vapor priming systems to promote resist adhesion for polysilicon, metals and SiO.sub.2 layers, and c) elevated temperature post-bake cycles. HMDS functions as an effective adhesion promoter for silicon and silicon oxide containing films, but provides effectively no surface-linking adhesion promotion with respect to silicon nitride films.
Accordingly, it would be desirable to develop alternate and improved techniques for providing better adhesion of photoresist to silicon nitride films.